[U-Boot] arm64 porting

[FengHua]

hi,
       The merging of arm64 to arm is complete. I have updated the "PATCH v4". There are many arguments about whether the porting should be made as a seperated architecture. now we can make comparison. 
       When porting as a seperated architecture, all architecture specific files and include files are located in arm64 directory. the code is simple and clear because any unused code could be got rid of. After merging it to arm, a few files are shared in which CONFIG_ARMV8 is used to distinguish arm64 specific part. That's ok except that there are two architecture code in arm architecture, ARM and ARM64(IH_ARCH_ARM and IH_ARCH_ARM64). 
       It's hard to say which implementation is better. All things depends on how u-boot want it's code to be.


Regards.
       David Feng